Amid disturbing reports that Bashar al-Assad may be preparing to use chemical agents against insurgent opposition, the Obama administration is reiterating its position that such a step constitutes a “red line” the Syrian dictator dares not cross. “Today I want to make it absolutely clear to Assad and those under his command: The world is watching,” President Obama said on Monday. “The use of chemical weapons is and would be totally unacceptable. And if you make the tragic mistake of using these weapons, there will be consequences and you will be held accountable.” Added Secretary of State Hillary Clinton: “we are certainly planning to take action” in such an event.

That seems like an uncontroversial position. Except when you consider that, in the nineteen months since the Syrian uprising began, Assad’s forces have killed the great majority of the more than 40,000 people to die in the conflict, many thousands of them innocent civilians. His men have indiscriminately shelled residential neighborhoods; tortured prisoners; allegedly burned infants in their beds and even buried men alive. All without crossing a red line.

So why draw a line at chemical weapons? “The international community has spelled out a specific set of rules and norms outlawing the use of chemical or biological weapons,” explains White House spokesman Tommy Vietor. “The death to civilians is indiscriminate and the human suffering they inflict is horrific.”

It certainly is. Assad is thought to have a stockpile of sarin gas, which, for instance, produces drooling, convulsions and vomiting before death. But conventional weapons are horrific, too. Explosives cause dismemberment, severe burns, collapsed lungs. And while the effects of poison gas are often likened to torture, countless Syrians have already endured torture–the kind with cables, whips and electric shocks to the genitals.

As Vietor notes, international law does treat chemical weapons differently. They were banned after World War I by the Geneva Protocol (which Syria signed), and again by the 1993 Chemical Weapons Convention. (Syria is among a small and nasty club of states not to join that one). But Assad’s brutality has violated international law many times over; chemicals aren’t a game-changer.

So the real issue here isn’t moral or legal. It’s psychological–and therefore political. Academics debate why chemical weapons carry a special stigma; it may be a special genetic aversion to poison, or perhaps a dread-inducing unfamiliarity with their effects. But the fact is that Assad’s use of chemical weapons would generate a degree of revulsion and outrage that would give the west the political cover required for direct intervention. “The possible use of chemical weapons would be completely unacceptable for the whole international community,” NATO Secretary-General Anders Fogh Rasmussen told reporters this week. The White House’s Vietor even invokes Saddam Hussein, whose gassing of the Kurds was an oft-repeated rationale for the 2003 invasion of Iraq. “The last leader to do so was Saddam Hussein, clearly a pariah in the international community,” adds Vietor. “Assad could still have some possible exit valves available to him. I can’t imagine any state would help him if he gassed his people.”

There is one other rationale for this red line, one that the White House doesn’t publicly emphasize: terrorism. Chemical weapons tend to be relatively portable, and give a terrorist the theoretical ability to kill large numbers of people in a dense urban area. (That point has attracted the particular interest of Israeli Prime Minister Benjamin Netanyahu, who is reportedly contemplating airstrikes against Syria’s chemical weapons depots.) Even so, when five members of the fanatical Japanese cult Aum Shinrikyo released sarin gas on the Japanese subway in March of 1995, the death toll of thirteen was a small fraction of the 190 people killed on the Madrid subway in 2006, when the method was old-fashioned high explosives.

Unless he runs out of conventional weapons, Assad would be foolish to incite America by tapping his chemical arsenal. He’s spent most of the past two years inflicting blood-curdling suffering on his people. There’s little reason to think we’ll try to interfere–so long as his sadism is the conventional kind, the kind we apparently can tolerate.